The first human subject of Neuralink experienced detachment of approximately 85% of the electrode connections within a few weeks after implantation. This incident is not an isolated case - the signal of invasive brain computer interface electrodes continues to decay for weeks to months after implantation, which is a common problem that has plagued the neuroscience community for decades.

The root of the problem lies not in the electronic design of the electrodes, but in the "mechanical war" between the electrodes and the brain. The Young's modulus of silicon is approximately 130-185 GPa, while the modulus of brain tissue is only 1-10 kPa. Mechanical mismatches of over 5 orders of magnitude result in micro motion damage at the electrode tissue interface with every breath, heartbeat, and head rotation. Injury triggers immune response: Microglia begin to respond within 30 minutes after implantation, while astrocytes activate and migrate in the following hours. After several weeks, a fibrous envelope composed of reactive astrocytes, connective tissue, and extracellular matrix - called glial scar - gradually forms around the device, physically isolating the electrodes from the neurons. At the same time, the disruption of the blood-brain barrier causes plasma proteins to adsorb onto the electrode surface, further exacerbating the inflammatory cascade reaction.

2、 From rigidity to flexibility: the mechanical adaptation logic of substrate selection

The core idea to solve this problem is to make the mechanical properties of the electrode as close as possible to brain tissue.

The Young's modulus of flexible polymer substrates is much lower than that of silicon. The Young's modulus of polyimide (PI) is about 8.37 GPa, and the tensile strength can reach 390 MPa, with high mechanical strength and good biocompatibility. In a comparative study, a flexible PI based electrode array showed more stable signal-to-noise ratio, single unit output, and impedance characteristics after implantation in the deep brain region of the mouse striatum; The performance of rigid silicon-based probes continues to deteriorate over time. Further histological analysis showed that the activation of microglia and IgG contamination within a 50 μ m radius around the flexible electrode were significantly lower than those in the rigid group.

Liquid crystal polymer (LCP) is another substrate that has attracted much attention. The water absorption rate of LCP is less than 0.04%, far lower than about 2.8% of PI and<1% of PDMS. Low water absorption rate means that after long-term immersion in cerebrospinal fluid environment, the dielectric properties and insulation characteristics of LCP will not deteriorate due to water infiltration. Accelerated immersion testing showed that the LCP encapsulation pattern survived for over 300 days in 75 ° C PBS, with an estimated equivalent lifespan of approximately 10 years under body temperature conditions.

Polyether ether ketone (PEEK) has advantages in scenarios that require higher mechanical strength. The microstructure of PEEK film can achieve directional and ordered arrangement in topological structure, which has unique value in the design of composite neural electrodes.

3、 Gold plating: Transforming flexible substrates from "insulating films" to "functional electrodes"

Flexible substrates provide a "skeleton" for mechanical adaptation, but the collection of neural signals ultimately depends on the conductive layer. Gold (Au) has become the core interface material for brain computer interface electrodes due to its unique physical and chemical properties.

Gold does not corrode or release toxic ions in physiological environments, and its chemical inertness ensures that the electrode maintains signal acquisition capability during implantation cycles lasting for several years. The resistivity of gold is about 2.44 μ Ω· cm, and its high conductivity ensures low loss transmission of microvolt level neural electrical signals in the brain. In addition, the ductility of gold enables it to bond with flexible substrates such as PI, PEEK, LCP, etc., with a Young's modulus of approximately 100-300 GPa. The structural design of flexible substrates can effectively alleviate mechanical mismatches.

In the comparative study of electrode materials, the gold electrode exhibits the lowest impedance - approximately 439.2 Ω± 5% at different surface areas, and has the highest gain in the high frequency range. This low impedance characteristic directly translates into higher signal-to-noise ratio and more accurate neural signal acquisition.

Advanced Institute of TechnologyIndependently built magnetron sputtering and vacuum evaporation production lines, using roll to roll continuous production technology, can achieve precise deposition of metal layers on flexible polymer film substrates such as PI, PET, FEP, LCP, PEEK, etc. The substrate width can reach 350 mm, and the ultimate vacuum degree can reach 1 × 10 ⁻⁴ Pa. Taking PI gold plating film as an example, the substrate thickness can be as thin as 5 μ m. The gold plating layer achieves stress transition from the substrate to the metal layer through gradient coating design, effectively avoiding interface peeling caused by modulus differences.

4、 Technological breakthrough: precise control from adhesion to porosity

Transforming the theoretical advantages of gold plating into reliable electrode products relies on precise control of core processes. In response to the engineering proposition of brain computer interface electrodes, Advanced Institute Technology has achieved triple process breakthroughs.

• Breakthrough in interface integration - plasma pretreatment.In response to the industry problem of insufficient bonding strength between metal and polymer substrates, plasma pretreatment technology effectively improves the bonding strength between the substrate and the metal layer by bombarding the substrate surface with plasma. The ion beam assisted deposition technology forms an atomic level bond between the gold coating and the PI substrate, which has been verified by the hundred grid test to meet the 5B level adhesion standard.
• Breakthrough in Coating Quality - Pulse Electroplating.Introducing pulse electroplating technology, by precisely controlling the switching time and frequency of the current, the porosity of the coating is effectively reduced, and the density and uniformity of the coating are improved - pulse electroplating can reduce the porosity of the coating to<0.5/cm ². Low porosity is crucial for long-term implantation: the micropores in the insulation layer and coating are channels for water vapor and ion penetration, which can lead to signal attenuation and electrical coupling.
• Breakthrough in Environmental Protection and Safety - Cyanide free Gold Plating.Adopting cyanide free gold plating technology has significant advantages in reducing environmental pollution and improving operational safety.

5、 Key Performance: What Engineers Should Pay Attention to

interface impedanceIt is the primary parameter that determines signal quality. The average impedance of the gold nanosheet modified electrode can be reduced from 4.5 k Ω to 0.85 k Ω at 1 kHz, a decrease of 81%. The ultra-thin gold micro electrode array exhibits excellent electrochemical performance of 0.73 Ω· cm ². Low impedance means higher signal-to-noise ratio and more accurate signal acquisition, which can significantly reduce common mode interference and motion artifacts.

long-term stabilityThis is the core requirement for implantable electrodes. The resistance change of the ultra-thin gold µ ECoG array after 50000 bending cycles is only 0.05%. The chemical inertness of gold endows gold-plated electrodes with excellent corrosion resistance, without significant attenuation in cerebrospinal fluid immersion environments.

6、 From 'Static' to 'Dynamic': Technological Evolution of Brain Computer Interface Electrodes

The brain computer interface electrode technology is undergoing a paradigm shift from "static" to "dynamic". Traditional implantable electrodes can only collect signals at fixed positions after implantation. In September 2025, the team of the Chinese Academy of Sciences Shenzhen Institute of Advanced Technology published the NeuroWorm achievement in the journal Nature - a flexible and driveable nerve fiber electrode with a diameter of only 196 μ m, which can "swim" in the brain and actively replace the monitoring target. The average thickness of the fiber envelope is less than 23 μ m 13 months after the electrode is implanted.

On May 18, 2026, China's first multi center clinical trial of a 128 channel fully implantable brain computer interface system was officially launched. It consists of two modules: cortical implantable flexible electrodes and highly integrated fully implantable signal collectors. The flexible electrodes are made of ultra-thin biocompatible materials, which can significantly reduce the immune response after implantation and accurately capture single neuron action potentials with high spatiotemporal resolution.

7、 Selection considerations: Engineering decisions from substrate to process

• The choice of substrate depends on the duration of implantation.LCP is preferred for scenarios that require long-term implantation (>10 years), as its ultra-low water absorption rate and accelerated lifespan data provide the longest equivalent lifespan support; PEEK can be chosen for scenarios that require higher mechanical strength and chemical stability; PI can be chosen for applications that are more sensitive to processing maturity and cost.
• The plating scheme is based on gold.Gold provides a comprehensive advantage of chemical inertness, low impedance, and biocompatibility. For electrodes that require electrical stimulation function, platinum or iridium oxide modification can be considered; For high-density recording arrays that require lower impedance, gold nanostructures or PEDOT: PSS coating modifications can be considered. Advanced Institute Technology has a deep accumulation in PEEK gold plating technology. The national standard GB/T 43763-2024 has clearly stipulated that various metal coatings such as nickel gold and copper nickel gold can be prepared on the surface of special non-metallic materials such as PEEK.
• The adhesion and porosity of the coating determine long-term reliability.The process threshold for ensuring that the coating does not peel off during repeated bending, implantation, and long-term service of the electrode is 5B level adhesion (ASTM D3359 highest grade) and porosity<0.5/cm ².
• The processing accuracy meets the requirements of high-density arrays.High channel count neural probes require micrometer level patterning accuracy. Advanced Institute Technology supports a full chain process from surface activation, vacuum coating, to metal coating, and can customize the coating system and microstructure of electrode materials according to different application needs.

8、 Conclusion

The essence of brain computer interface electrodes is to establish a stable, low-noise, and biologically friendly signal channel between rigid electronic devices and soft neural tissue. It matches the mechanical properties of brain tissue with flexible substrates (PI, LCP, PEEK), ensures signal quality with gold plating, and converts the chemical inertness of gold into reliable long-term performance through plasma pretreatment and pulse electroplating processes - all three work together to answer a core engineering question: how to make electronic devices as soft as brain tissue while also understanding the language of neurons.

From rigid silicon-based electrodes to flexible polymer based electrodes, from static implantation to dynamic "migration" - the technological form of brain computer interface electrodes is constantly evolving, but the position of coating as the core interface engineering has remained unchanged. Understanding the failure mechanisms of mechanical mismatches, the chemical and mechanical boundaries of different substrates, and the decisive impact of gold plating processes on electrode performance - these are the key to upgrading brain computer interface electrodes from "cutting-edge research tools" to "clinical medical devices".